Generally, vehicles include a subframe below left and right front side skeleton members, and a power source and left and right front suspensions are supported by the subframe. As such a subframe, a subframe is disclosed which is formed (by casting) of aluminum alloy to attain weight reduction of the vehicle. For example, in Patent Literature 1, the subframe formed of aluminum alloy has a plurality of reinforcing ribs formed on an inner surface thereof in a saw-toothed shape as a whole to thereby securing strength and rigidity.
For casting the subframe using a mold, spaces or cavities for forming the reinforcing ribs are formed inside the mold. The cavities for the reinforcing ribs are used as guide passages of molten metal. By using the cavities for the reinforcing ribs as the guide passages, molten metal of aluminum alloy can be guided through all the regions of the subframe.
In Patent Literature 1, however, since the reinforcing ribs are formed in the saw-toothed shape, the cavities for the reinforcing ribs, which are formed inside the mold, have a saw-toothed shape. Therefore, when casting the subframe, it is difficult to guide smoothly the molten metal of aluminum alloy along the cavities for the reinforcing ribs, so that it is difficult to maintain a good flow.